Silicon nitride films are used as insulating films, protection films, and so forth in various semiconductor device. It is known that a silicon nitride film of this kind can be formed by plasma CVD or the like using a silicon-containing compound gas, such as silane (SiH4), as a source gas and a nitrogen-containing compound gas, such as nitrogen or ammonia.
In a plasma CVD apparatus for performing plasma CVD as described above, as the film formation is repeated on target substrates, silicon nitride is deposited on portions inside the chamber other than the target substrates. If extraneous deposits thus formed on portions inside the chamber exceed a certain film thickness, the film formation performed in the plasma CVD apparatus is adversely affected. For example, the extraneous deposits formed on portions inside the chamber affect gas flows and/or thermal distribution inside the chamber, and may deteriorate the film formation rate and/or film formation uniformity. Further, particles may be generated from the extraneous deposits and deposited on the target substrate. In light of the problems described above, it is necessary to periodically perform cleaning inside the chamber to remove the extraneous deposits formed on portions inside the chamber. For this purpose, there has been proposed a plasma cleaning method for etching the interior of the chamber by use of plasma of a reactive gas containing a fluorine compound, such as NF3, (for example, Jpn. Pat. Appln. KOKAI Publication No. 7-201738).
As disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-201738, plasma cleaning is typically arranged to use a cleaning gas (such as NF3) of the type having an etching effect relative to extraneous deposits on portions inside the chamber and to dry-etch the extraneous deposits by plasma of the cleaning gas. However, where a fluorine-containing gas, such as NF3 gas, is used for cleaning, it is necessary to remove residual fluorine inside the chamber because the residual fluorine adversely affects a film formation process subsequently performed. Accordingly, this patent document 1 describes such that a process using a gas containing NF3 is followed by a stage of removing fluorine inside the chamber by use of plasma of a reducing gas containing NH3.
The technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-201738 described above is arranged to use a stage of removing residual fluorine inside the chamber after NF3 cleaning, but cannot be said to involve sufficient studies concerning conditions for performing the cleaning process at high throughput to swiftly remove residual fluorine while attaining a cleaning effect practically sufficient.
Further, where a cleaning process includes a plurality of stages, as in the technique described above, it is difficult to find the endpoints of the respective stages. If the cleaning is excessively performed due to erroneous determination of the endpoints, plasma of a cleaning gas etches members inside the chamber, such as a worktable (susceptor) for placing a target substrate thereon and a cover ring for guiding the target substrate, and causes plasma damage thereon, which shortens the service life of the members. Particularly, if the cleaning is performed for a longer time to enhance the cleaning effect, the plasma damage becomes larger and thereby deteriorates the throughput. In reverse, if conditions are set to suppress the plasma damage and thereby increase the throughput, cleaning becomes insufficient, thereby generates particles, and adversely affects the film formation.